Rebound, its nature and effect on engineering works

Abstract

Summary Rebound of rock masses, defined as the expansive recovery of surficial crustal material, either instantaneously, time-dependently, or both, initiated by the removal or relaxation of superincumbent loads, is found in most geological terrains. The applied loads resulting from past or present geological processes are removed or relaxed by (1) natural processes such as valley erosion (long term) or (2) artificial processes such as excavations (short term). Rebound features are most pronounced if the time period of load removal is short compared to the relaxation time of the material, assuming that relaxation time is a material property. Some factors that affect rebound and its intensity are: Initial fabric, lithology, crack density, anisotropy, density and moisture content. Magnitudes of deviatoric stresses (stress differences, stress ratios) operating at the time of load removal, their orientation with respect to the free surface and their rate of unloading. Initial strength and time-dependent strength. Environmental conditions, such as moisture, temperature and other weathering agents. The near-surface stress distribution and time-dependent strain release measured within different geological terrains suggest that the redistribution and/or release of stored-strain energy within rock masses may be a major factor contributing to rebound. Strains, stored in the rocks as a result of past and present geological loads, can be released by excavation processes. As new surfaces are formed, the rock mass changes volume, creating physical property changes that include increased void space and higher permeabilities, thus allowing more rapid access for chemical agents that accelerate the weathering process. Rebound is a rate-dependent process that is manifested differently in various geological terrains. Long-term stability is dependent on changing fracture density and the rate at which strength decreases within near-surface rock material. The strength reduction may be, among other things, a function of the internal decay of cohesion due to the development or propagation of fractures. The time-dependent processes of rebound and the ambient stress fields are important in the development of criteria for the long-term design of engineering installations that are placed within or on disturbed rock masses.